MARIUPOL, Ukraine — When the Ukrainian navy command ship was close enough to Russia’s shore to get the country’s state television channels, Capt. Oleksandr Hryhorevskyi switched on the news and wondered: Why all the fuss about his rusty old boat?

The appearance of his more-than-50-year-old vessel — aptly named Donbas, for the eastern region locked in an eight-year conflict between Ukraine’s armed forces and Moscow-backed separatists — dominated Russian broadcasts one night this month. With an estimated 100,000 Russian troops massed on Ukraine’s border, the near-confrontation between Russian forces and the Donbas in the Sea of Azov could have been the spark for war on the ground.

On Ukraine’s front, a real war overshadows worries about a possible future one with Russia

Russia’s main domestic intelligence agency, the FSB, accused the Donbas of navigating toward the Kerch Strait, the waterway between mainland Russia and annexed Crimea, which the ship hadn’t filed for permission to pass. The FSB said the Donbas ignored its instructions to correct course — and Kremlin propagandists quickly labeled the ship’s action a “provocation.”

Margarita Simonyan, editor in chief of the government-funded television channel RT, banged the war drum loudest on Twitter. “Has it started? Mother, don’t let me down,” she wrote.

The flare-up ended when the Donbas turned around, less than three nautical miles from the Kerch Strait. But the incident, though ultimately harmless, underscored a strategic weakness for Ukraine’s military in a tinderbox for war.

The Sea of Azov, a shallow internal body of water, smaller than Lake Michigan and bounded by Russia and Ukraine, is the one place the two countries’ forces risk direct conflict daily. It’s packed with Russian ships — there are about four times as many of them as there are Ukrainian vessels, according to Andrii Klymenko, the Kyiv-based editor in chief of the Black Sea News. And because there is no maritime border in the sea, the Russian fleet can get as close to the Ukrainian shore as it wishes, arguably making Ukraine’s southeastern seaside cities the most vulnerable in the country.

U.S. intelligence has warned that the Kremlin could be planning a multipronged military attack against Ukraine as soon as early next year. For all the international attention paid to the concentration of Russian troops on Ukraine’s land borders, if an invasion originates from the sea, Ukrainian forces are largely powerless. They cannot legally take any action against until the first Russian soldier steps on land.

Meanwhile, Ukraine’s fleet is limited in its movement on the Azov by what Kyiv says is Russia’s regular practice of restricting some zones at little notice for what it claims are artillery training exercises. The Ukrainian navy said this month that nearly 70 percent of the Azov was blocked for transport because of navigation warnings from Moscow.

“In terms of security, the Sea of Azov is dominated by Russia,” Foreign Minister Dmytro Kuleba told The Washington Post. “And in case of war, it will be heavily used by them to put pressure on our southern cities on the Azov shore.”

“Under the current circumstances,” Kuleba said, “of course Russia dictates the situation in the Azov and basically uses it as a war theater.”

Hryhorevskyi said the Donbas was anchored on Dec. 8 when he received a Russian navigation warning that the zone it was in was closing. That left him nowhere to go but in the direction of the Kerch Strait, he said. At around 1:30 p.m. on Dec. 9, a Russian ship contacted the Donbas to remind it that passage through the waterway required 48 hours’ notice. Hryhorevskyi said the Donbas responded that it did not plan to sail through the Kerch Strait. He said the vessel was unarmed.

The Russian ship then asked for the Donbas’s plans. Hryhorevskyi said he declined to reveal them. They repeated the exchange four times, he said. By evening, the Donbas was flanked by three Russian ships with little room to maneuver, Hryhorevskyi said. He warned the Russians to back off as the Donbas turned around.

Hryhorevskyi said a Russian attack aircraft flew over the Donbas and one of the Russian ships tailing it threatened to open fire if it didn’t reverse course.

The next day, Russian Foreign Ministry spokeswoman Maria Zakharova told state television that the Donbas represented a “new provocation at sea” that was part of a string of “provocative actions that are being carried out around our borders.” In recent weeks, Russian officials have claimed that it is Ukraine and NATO that are threatening Russia at its border.